When oxygen levels in polluted rivers are modeled, the effects of aquatic plant respiration and photosynthesis are usually represented by a single term. Such a simple approach suffers the disadvantage that it does not permit the relative contributions of the different communities of plants to be separately assessed. A more flexible approach is described in which the effect of three important plant communities on the dissolved oxygen levels is represented by a separate term, each of which is dependent on the measured or estimated biomass of that community. The calibration of such a model is described for the Waikato River, New Zealand, where the phytoplankton were shown to be the principal contributors to the oxygen production and consumption. The model is then used to predict the effects of the discharge of cooling water from a proposed power station on the dissolved oxygen levels in the river.